Can fastmail's alias send email fron email client?
 

I setup alias in the web options, the alias can send email from webmail.
Can I setup email clients like thunderbird to send mails with alias?

Thanks.

Yes

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How to do it?

It depends on the client.

In general I would think you set up the account in your client the same way you do for your actual FM account but in addition add the info needed to spoof the sender address to be the alias (for example the identity information in the case of Thunderbird).

In Thunderbird's case alias's are called multiple identities. You can chose an alias when sending by selecting it from the From: drop down list. How to add another identity:

Go to your account settings.
Choose the account to which you wish to add additional identities.
Click the "Manage Identities..." button (on the lower right).
Click the "Add..." button and then add the necessary information for the additional identity for the account.
Repeat as necessary for additional identities.

http://kb.mozillazine.org/Multiple_i...e-mail_account

The details and terminology vary for other email clients, but its a common feature.

Thank you. I will try that.